The present disclosure is related to lithography, and more particularly to the design and manufacture of a surface which may be a reticle, a wafer, or any other surface, using charged particle beam lithography.
In the production or manufacturing of semiconductor devices, such as integrated circuits, optical lithography may be used to fabricate the semiconductor devices. Optical lithography is a printing process in which a lithographic mask or photomask manufactured from a reticle is used to transfer patterns to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays or even other reticles. Also, extreme ultraviolet (EUV) or X-ray lithography are considered types of optical lithography. The reticle or multiple reticles may contain a circuit pattern corresponding to an individual layer of the integrated circuit and this pattern can be imaged onto a certain area on the substrate that has been coated with a layer of radiation-sensitive material known as photoresist or resist. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits may then be separated from one another by dicing or sawing and then may be mounted into individual packages. In the more general case, the patterns on the substrate may be used to define artifacts such as display pixels or magnetic recording heads.
In the production or manufacturing of semiconductor devices, such as integrated circuits, maskless direct write may also be used to fabricate the semiconductor devices. Maskless direct write is a printing process in which charged particle beam lithography is used to transfer patterns to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays, imprint masks for nano-imprinting, or even reticles. Desired patterns of a layer are written directly on the surface, which in this case is also the substrate. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Some of the layers may be written using optical lithography while others may be written using maskless direct write to fabricate the same substrate. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits are then separated from one another by dicing or sawing and then mounted into individual packages. In the more general case, the patterns on the surface may be used to define artifacts such as display pixels or magnetic recording heads.
Two common types of charged particle beam lithography are variable shaped beam (VSB) and character projection (CP). In VSB charged particle beam lithography, a precise electron beam is shaped and steered so as to expose a resist-coated surface, such as the surface of a wafer or the surface of a reticle. These shapes are simple shapes, usually limited to rectangles of certain minimum and maximum sizes and with sides which are parallel to the axes of a Cartesian coordinate plane, and triangles with their three internal angles being 45 degrees, 45 degrees, and 90 degrees of certain minimum and maximum sizes. At pre-determined locations, doses of electrons are shot into the resist with these simple shapes. The total writing time for this type of system increases with the number of shots. In CP charged particle beam lithography, there is a stencil in the system that has in it a variety of shapes which may be rectilinear, arbitrary-angled linear, circular, annular, part circular, part annular, or arbitrary curvilinear shapes, and which may be a connected set of complex shapes or a group of disjointed sets of a connected set of complex shapes. An electron beam can be shot through the stencil to efficiently produce more complex patterns (i.e. characters) on the reticle. In theory, such a system can be faster than a VSB system because it can shoot more complex shapes with each time-consuming shot. Thus, an E shot with a VSB system takes four shots, but the same E can be shot with one shot with a character projection system. Note that VSB systems can be thought of as a special (simple) case of character projection, where the characters are just simple characters, usually rectangles or 45-45-90 triangles. It is also possible to partially expose a character. This can be done by, for instance, blocking part of the particle beam. For example, the E described above can be partially exposed as an F or an I, where different parts of the beam are cut off by an aperture.
The photomasks used for optical lithography are manufactured from reticles onto which a pattern has been formed. There are a number of technologies used for forming patterns on a reticle, including optical lithography and charged particle beam lithography. The most commonly-used system is a VSB charged particle beam system. Reticle writing typically involves multiple passes whereby the given shape on the reticle is written and overwritten. Typically, two to four passes are used to write a reticle to average out precision errors in the charged particle beam system, allowing the creation of more accurate photomasks. Conventionally, within a single pass the constituent shapes do not overlap. Multi-pass writing has a disadvantage of increasing the total time required for the charged particle beam system to form the pattern on the reticle. This extra time increases the cost of the reticles and the resulting photomasks. At present, no available CP charged particle beam system is suitable for use in making photomasks.
When using charged particle beam lithography either for making reticles or for direct write, individual doses or shots of charged particles are conventionally designed to avoid overlap wherever possible, and for multi-pass writing, to avoid overlap within a single pass. The dosage is assumed to be the same, or “normal,” at all parts of the formed pattern. This greatly simplifies calculation of how the resist on the reticle will register the pattern. Because of the assumed normal dosage, the fracturing programs that assign VSB shots conventionally do not output dosage information.
The cost of charged particle beam lithography is directly related to the time required to expose a pattern on a surface, such as a reticle or wafer. The exposure time is related to the number of shots required to produce the pattern. Patterns can often be formed in fewer shots if the shots are allowed to overlap. Additionally, patterns can be formed in fewer shots if the union of shots is allowed to deviate from the target pattern. When these techniques are used, calculation of the pattern that will be registered by the resist is more complicated. Charged particle beam simulation may be used to determine the pattern that will be registered by the resist. Charged particle beam simulation, which may include simulation of various charged particle beam writing and resist effects, is a compute-intensive process, however. It is impractical to simulate the pattern for an entire integrated circuit, and then to re-simulate the pattern every time a proposed charged particle beam shot is changed.
It would therefore be advantageous to be able to easily determine how resist on a surface such as a wafer or reticle will register a pattern formed by a plurality of charged particle beam shots. This would enable the use of overlapping shots and variable shot dosages. With overlapping shots and variable dosages, patterns can be formed on a surface with fewer shots, thus reducing the cost of forming the pattern on a surface such as a reticle or a wafer, and consequently reducing the cost of manufacturing photomasks and semiconductor devices.